A diode is one of the most straightforward types of semiconductor devices and draws its name from its simple two-electrode structure. Diodes act as one way barriers to the passage of electrons because they permit electrons to flow in one direction, but bar their passage in the opposite direction under normal operating conditions. Typical applications for semiconductor diodes include rectification, the conversion of alternating current to pulses of direct current; clamping, the prevention of voltage in one wire from exceeding the voltage in a second wire; demodulation, the modification of a wave signal; and the production of logic gates, switching circuit building blocks which utilize "yes" and "no" statements as inputs to make certain simple decisions with the answer also expressed as "yes" or "no." Diodes are accordingly fundamental elements in modern electronic devices of all types.
Typically, the performance of diodes are characterized using five basic characteristics which are well understood in this art: forward current (I.sub.f); forward voltage (V.sub.f); reverse current (I.sub.r); reverse breakdown voltage (V.sub.br); and reverse recovery time (T.sub.rr).
One candidate material for diodes with high quality characteristics is silicon carbide (SiC). Silicon carbide has a number of theoretical advantages for diodes and particularly for rectification. These include a wide band gap, a high thermal conductivity, a low dielectric constant, a high saturated electron drift velocity, a high breakdown electric field, a low minority carrier lifetime, and a high dissociation temperature. Taken together, these properties have long indicated that semiconductor devices formed from silicon carbide should be operable at much higher temperatures than devices made from other semi-conductor materials as well as at higher speeds and higher power levels.
For example, semiconductor devices used in applications as diverse as deep-well drilling and jet aircraft engines both require devices that can operate at temperatures of 200.degree. C. and 350.degree. C. respectively, or even higher. Additionally, however, devices in such applications must operate over wide temperature ranges as well, for example from below 0.degree. C. to 350.degree. C. To date, any devices available for such applications must be appropriately cooled--often a complicated engineering task--or are simply unavailable given the temperature limitations of common semiconductor materials such as silicon (Si).
Nevertheless, in spite of the long-standing recognition of the theoretical advantages offered by the electronic properties of silicon carbide, rectifying diodes and other semiconductor electronic devices formed from silicon carbide are just now beginning to reach the commercial marketplace. The lack of prior success in producing silicon carbide devices has resulted from the difficulties encountered in working with silicon carbide. Silicon carbide is an extremely hard material, often used as an abrasive. It must be worked at extremely high temperatures under which other materials cannot be worked, and crystallizes in well over 150 polytypes. Many of these polytypes are in turn separated by rather small thermodynamic differences. For these reasons, production of monocrystalline thin films of silicon carbide that are necessary for certain devices, and production of large single crystals of silicon carbide each having appropriate single polytypes, which are useful as substrate material and for other applications, have remained elusive goals. Additionally, certain doping techniques which have been successfully developed for other materials have proven unsuccessful when used in conjunction with silicon carbide.
Recently, however, and as set forth in co-pending applications Ser. No. 07/284,035, filed Dec. 14, 1988 and Ser. No. 07/403,690, filed Sept. 6, 1989 both to Edmond for "Ultra-Fast High Temperature Rectifying Diode Formed in Silicon Carbide," techniques have been developed for producing appropriate rectifying diodes in silicon carbide with particularly favorable characteristics. These applications are incorporated entirely herein by reference.
As set forth in these co-pending applications, the resulting silicon carbide devices can operate at extremely high temperatures; i.e. 350.degree. C. or higher, temperatures at which devices formed from conventional semiconductor materials such as silicon, gallium arsenide, or indium phosphate, may become inoperable, melt, or even vaporize.
From a practical and commercial standpoint, the successful operation of such a silicon carbide device at such temperatures must be accompanied by an appropriate operational reliability. Such reliability depends upon a number of factors. These include: the mechanical stability of the semiconductor device and its housing, appropriate and reliable ohmic contacts to the device, reliable metallurgical bonds between contacts, the maintenance of the device and its features in original condition even under adverse operating conditions, the use of an air-tight envelope to prevent infliction of mechanical damage on the semiconductor material and to isolate it from harmful impurities in the surrounding environment or atmospheric air, and the use of appropriate passivating and protecting coatings. In particular, for a device formed from silicon carbide that is intended to operate at high temperature, the metallic contacts in the device must be carefully matched to avoid breakage during thermal cycling.
There thus exists the need for an appropriate packaging technology that will both enable and maintain the long term, reliable use of high temperature diodes both during high temperature operation and during excursions over wide temperature ranges.
One packaging technique common in the semiconductor industry is the so-called "double slug" diode. The double slug diode was developed to overcome the various shortcomings and power limitations of diodes that were constructed by soldering fine wire leads to the opposite faces of a small semiconductor "die" such as a junction diode. A double slug diode is typically formed of cylindrical portions of molybdenum (Mo) or tungsten (W) which in the prior art use with silicon have been coated with a protective layer of gold, silver or other noble metal. A semiconductor device such as a p-n junction diode is placed between the slugs. The slugs and the diodes sandwiched between them can then be packaged and sealed in plastic or glass. Wire leads are conveniently attached to the slugs (and exhibit greater structural stability) much more easily than fine wires can be attached directly to a diode. The wire leads in turn are used to add the resulting device to an intended circuit or other application.
To date, however, because most such diodes are formed of materials which operate at relatively low temperatures, and for which "high" temperatures are considered to be on the order of 150.degree. C., the packaging materials and techniques used to form such devices are often disadvantageous and sometimes entirely useless in packaging a device formed from silicon carbide which must operate at much higher temperatures; e.g. 350.degree. C. and above; and over correspondingly wide temperature ranges; e.g. -65.degree. C. to about 350.degree. C. or above.
One particularly important element is the method and material used to join the leads to the appropriate slugs. Some devices are held together simply by a glass or plastic packaging material. Others use joining materials such as solders or brazes to connect the leads to the slugs. Packaging the silicon carbide diodes of the present invention without any joining material is generally unsatisfactory because of the stresses that result from the wide temperature ranges over which the devices must operate. Conventional brazes or solders that are otherwise suitable for devices operating at ordinary temperatures can be unsatisfactory for operating conditions that include the temperatures at which the silicon carbide diodes of the present invention can operate. An appropriate joining material must have and maintain a high strength and a high power carrying capability at operating temperatures of 350.degree. C. or greater and over time temperature cycles that include repetitive excursions between temperatures of -65.degree. C. and 350.degree. C. or more.
Typical solders have melting points well below 350.degree. C. Common lead-tin (Sn-Pb) solders melt at about 179.degree.-188.degree. C. Gold-tin (Au-Sn) solders melt at about 280.degree. C., and tend to form brittle joints. Brazes with higher melting points are available, e.g. gold-copper (Au-Cu, 780.degree. C.), but also exhibit brittleness when operating temperatures of 350.degree. C. are experienced. A lead-silver-tin solder is useful for some higher temperature operations, but has poor corrosion resistance. These and other properties of materials are discussed in D. Fink, "Electronics Engineers' Handbook" (Second Edition, 1982), Section 6.
There thus exists the need for a packaging technique and resulting package for a diode formed of silicon carbide for which all of the packaging elements are suitable for operation at temperatures above 200.degree. C. and during temperature excursions at -65.degree. C. and at least 350.degree. C.